udf: Fix deadlock in udf_release_file()

udf_release_file() can be called from munmap() path with mmap_sem held.  Thus
we cannot take i_mutex there because that ranks above mmap_sem. Luckily,
i_mutex is not needed in udf_release_file() anymore since protection by
i_data_sem is enough to protect from races with write and truncate.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Reviewed-by: Namjae Jeon <linkinjeon@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>