mutex_unlock() later in seq_lseek()

All manipulations with struct seq_file::version are done under
struct seq_file::lock except one introduced in commit
d6b7a781c51c91dd054e5c437885205592faac21
aka "[PATCH] Speed up /proc/pid/maps"

Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>