ext4: allow a NULL argument to ext4_ext_drop_refs()

Teach ext4_ext_drop_refs() to accept a NULL argument, much like
kfree().  This allows us to drop a lot of checks to make sure path is
non-NULL before calling ext4_ext_drop_refs().

Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --cc fs/ext4/extents.c
Simple merge

diff --cc fs/ext4/extents_status.c
Simple merge

diff --cc fs/ext4/migrate.c
Simple merge

diff --cc fs/ext4/move_extent.c
Simple merge