ext4: fix duplicated mnt_drop_write call in EXT4_IOC_MOVE_EXT

Caused, AFAICS, by mismerge in commit ff9cb1c4

 ("Merge branch
'for_linus' into for_linus_merged")

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org  # 3.3+
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>