fs: update documentation of get_write_access() and friends

}

/*

 * This is used for regular files where some users -- especially the

 * currently executed binary in a process, previously handled via

 * VM_DENYWRITE -- cannot handle concurrent write (and maybe mmap

 * read-write shared) accesses.

 *

 * get_write_access() gets write permission for a file.

 * put_write_access() releases this write permission.

 * This is used for regular files.

 * We cannot support write (and maybe mmap read-write shared) accesses and

 * MAP_DENYWRITE mmappings simultaneously. The i_writecount field of an inode

 * can have the following values:

 * 0: no writers, no VM_DENYWRITE mappings

 * < 0: (-i_writecount) vm_area_structs with VM_DENYWRITE set exist

 * > 0: (i_writecount) users are writing to the file.

 * deny_write_access() denies write access to a file.

 * allow_write_access() re-enables write access to a file.

 *

 * The i_writecount field of an inode can have the following values:

 * 0: no write access, no denied write access

 * < 0: (-i_writecount) users that denied write access to the file.

 * > 0: (i_writecount) users that have write access to the file.

 *

 * Normally we operate on that counter with atomic_{inc,dec} and it's safe

 * except for the cases where we don't hold i_writecount yet. Then we need to