lock_may_read — checks that the region is free of locks
the inode that is being read
the first byte to read
the number of bytes to read
Emulates Windows locking requirements. Whole-file mandatory locks (share modes) can prohibit a read and byte-range POSIX locks can prohibit a read if they overlap.
N.B. this function is only ever called from knfsd and ownership of locks is never checked.