fsnotify: Fix misspelling of "writable"

Several file system notification system headers have "writable"
misspelled as "writtable" in the comments. This patch fixes it in the
fsnotify header.

Signed-off-by: Vicki Pfau <vi@endrift.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20240306020831.1404033-2-vi@endrift.com>