f2fs: split f2fs_d_compare() from f2fs_match_name()

[ Upstream commit f874fa1c7c7905c1744a2037a11516558ed00a81 ]

Sharing f2fs_ci_compare() between comparing cached dentries
(f2fs_d_compare()) and comparing on-disk dentries (f2fs_match_name())
doesn't work as well as intended, as these actions fundamentally differ
in several ways (e.g. whether the task may sleep, whether the directory
is stable, whether the casefolded name was precomputed, whether the
dentry will need to be decrypted once we allow casefold+encrypt, etc.)

Just make f2fs_d_compare() implement what it needs directly, and rework
f2fs_ci_compare() to be specialized for f2fs_match_name().

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>