I bought a 15W led bulb which is the equivalent of a 120W incandescent. But, I can't find a cord grip ceiling bulb holder rated higher than 100W.
Is it safe to put the 120W equivalent bulb in a 100W ceiling socket?
Yes, that is fine
"like" /"equivalent", etc. are based on light output.
Limitations on fixtures is based on heat output.
Some power (Watts) becomes light, the rest becomes heat. LEDs are more efficient and produce more light, and therfore less heat, than incandescent bulbs. Fixtures are rated on worst case scenarios. But a 15 W LED bulb can only put out 15 W heat. No problem.
It may be that fixture power rating is based on efficiency and expected heat output of standard incandescent bulbs, rather than total power output. But that would make very little difference. A quick search finds incandescent efficiency in the range, at best, of 5% to 10%. If a 100W incandescent bulb is 10% efficient (very optimistic) then 90% of the power - 90W - is lost as heat. If the 15W LED produced entirely heat and no light at all (which wouldn't be too useful), it would only produce 15W of heat. No contest.
All that being said, there can be heat issues with LED lighting. Generally this is with new fixtures designed specifically for LEDs, not putting LEDs into standard Edison base fixtures. In fact, some high-power fixtures include fans to dissipate the excess heat. This is because (a) an LED can be extremely compact as there is no functional need for a large "globe" around a filament (when there is a "filament" in an LED bulb, that's just for show) and (b) the driver circuitry between the incoming AC power and the actual LEDs produces heat. This is also the reason why some LED bulbs will specify "don't use upside down" - heat rises and if the heat is all generated at the bottom of the bulb and you install it upside down so that the bottom (as designed) is at the top, heat may not dissipate properly.
Two things to consider:
(1) is not going to happen. 100W incandescent bulb is ~95W heat source. 15W LED lamp is ~11W heat source. No problem here.
(2) may be a problem, or maybe not. As the incandescent lamp generates 8x the heat, it will rise its own temperature above the environment roughly 8 times more than the LED lamp. But, LED lamps are also much less tolerant to high temperatures.
If we consider the incandescent lamp glass heating up to 140C in a hot day (40C in the room, lamp 100C above the environment), it will be OK. A LED in the same conditions will heat 100/8 = 12.5C above the environment, e.g. 52.5C . Not a big deal, but probably not an optimal temperature for a LED either.
In the worst case, you'll get some less lifetime from the LED lamp.